This invention relates to the preparation of diaryl carbonates, and more particularly to their preparation using a catalyst system having improved homogeneity.
Diaryl carbonates are valuable intermediates for the preparation of polycarbonates, for example by transesterification with bisphenols in the melt. This method of polycarbonate preparation has environmental advantages over methods which employ phosgene, a toxic gas, as a reagent and environmentally detrimental chlorinated aliphatic hydrocarbons such as methylene chloride as solvents.
Various methods for the preparation of diaryl carbonates by an oxidative carbonylation (hereinafter sometimes simply xe2x80x9ccarbonylationxe2x80x9d for brevity) reaction of hydroxyaromatic compounds with carbon monoxide and oxygen have been disclosed. In general, the carbonylation reaction requires a rather complex catalyst. Reference is made, for example, to U.S. Pat. No. 4,187,242, in which the catalyst comprises a Group 8, 9, or 10 metal having an atomic number of at least 44 (hereinafter xe2x80x9cheavy transition metalsxe2x80x9d), said metals consisting of ruthenium, rhodium, palladium, osmium, iridium and platinum, or a complex thereof.
The production of carbonates may frequently be improved by including a lead-containing cocatalyst along with the heavy transition metal catalyst. Suitable lead-containing cocatalysts have been described broadly in various patents and publications, particularly in U.S. Pat. No. 5,498,789, which is incorporated herein by reference. Also required in general is the use of various halides, as illustrated by tetra-n-butylammonium bromide or chloride, as part of the catalyst package. Compounds characterized as inert solvents, such as toluene, diethyl ether, diphenyl ether and acetonitrile, can also be present.
The presence in the catalyst composition of lead compounds such as lead(II) oxide or lead(II) alkoxides or phenoxides, conventionally used as lead sources, frequently introduces problems of various kinds. Such compounds may interact with other materials present and frequently form precipitates. These precipitates, of variable composition, introduce irreproducibility into the carbonylation reaction and unpredictably affect yield and/or selectivity. They also adversely affect isolation and purification of the diaryl carbonate and impair recycle of catalyst constituents.
It is of interest, therefore, to develop improved methods of delivering lead to the carbonylation catalyst system.
The present invention provides lead-containing catalyst systems of improved properties, including an increase in homogeneity and in predictability of catalytic results.
In one of its aspects, the invention is a method for preparing a diaryl carbonate which comprises contacting at least one hydroxyaromatic compound with oxygen and carbon monoxide in the presence of an amount effective for carbonylation of a catalyst composition comprising the following and any reaction products thereof:
(A) a Group 8, 9, or 10 metal having an atomic number of at least 44 or a compound thereof,
(B) at least one bromide or chloride salt, and
(C) at least one lead compound having direct lead-carbon linkages.
Another aspect of the invention is a catalyst composition comprising components A-C as defined above.
Any hydroxyaromatic compound may be employed in the method of the present invention. Monohydroxyaromatic compounds, such as phenol, the cresols, the xylenols and p-cumylphenol, are generally preferred with phenol being most preferred. The invention may, however, also be employed with dihydroxyaromatic compounds such as resorcinol, hydroquinone and 2,2-bis(4-hydroxyphenyl)propane or xe2x80x9cbisphenol Axe2x80x9d, whereupon the products are polycarbonate oligomers.
Other essential reagents in the diaryl carbonate preparation method of the invention are oxygen and carbon monoxide, which react with the phenol to form the desired diaryl carbonate. They may be employed in high purity form or diluted with another gas such as nitrogen, argon or carbon dioxide which has no negative effect on the reaction.
For the sake of brevity, the constituents of the catalyst system of the invention are defined as xe2x80x9ccomponentsxe2x80x9d irrespective of whether a reaction between said constituents occurs before or during the carbonylation reaction. Thus, the catalyst system may include said components and any reaction products thereof.
Component A of the catalyst system is one of the heavy transition metals, preferably palladium, or a compound thereof. Thus, useful palladium materials include elemental palladium-containing entities such as palladium black, palladium/carbon, palladium/alumina and palladium/silica; palladium compounds such as palladium chloride, palladium bromide, palladium iodide, palladium sulfate, palladium nitrate, palladium acetate and palladium 2,4-pentanedionate; and palladium-containing complexes involving such compounds as carbon monoxide, amines, nitriles, phosphines and olefins. Preferred in many instances are palladium(II) salts of organic acids, most often C2-6 aliphatic carboxylic acids, and palladium(II) salts of xcex2-diketones. Palladium(II) acetate and palladium(II) 2,4-pentanedionate, especially the latter, are generally most preferred. Mixtures of the aforementioned palladium materials are also contemplated.
Component B is at least one bromide or chloride salt. It may be an alkali metal or alkaline earth metal bromide or chloride, preferably a bromide such as lithium bromide, sodium bromide, potassium bromide, calcium bromide or magnesium bromide. It may also be an onium bromide or chloride, preferably a bromide, including trialkylamine hydrobromides and tetraalkylammonium, tetraalkylphosphonium, hexaalkylguanidinium, and sulphonium bromides. Illustrative examples include tetramethylammonium bromide, tetraethylammonium bromide or tetra-n-butylammonium bromide; a tetraalkylphosphonium salt such as tetramethylphosphonium bromide; or a hexaalkylguanidinium salt such as hexaethylguanidinium bromide. The bromides are often preferred, with tetraalkylammonium bromides being especially preferred.
Component C is at least one lead compound having at least one direct lead-carbon linkage; i.e., an organolead compound. Illustrative organolead compounds are tetraethyllead and tetraphenyllead. Tetraethyllead is, of course, commercially available and has been employed as an anti-knock additive for motor fuels, especially gasoline, although, it is no longer employed for this purpose in the United States by reason of its toxicity.
The use of an organolead compound has significant advantages in improvement of homogeneity of the catalyst composition. Specifically, mixtures of organolead compound, phenol and ionic bromides or chlorides, preferably alkali metal or alkaline earth bromides or chlorides, or onium salts such as tetraalkylammonium and tetraalkylphosphonium bromides or chlorides are homogeneous, in contrast to similar mixtures containing lead(II) oxide, lead(II) alkoxides or lead(II) phenoxide. In preferred embodiments the ionic bromide or chloride is substantially soluble in hydroxyaromatic compound. In embodiments where substantial solubility of ionic bromide or chloride may not be obtained, then a co-solvent may be added, such as tetraglyme or other polyether as taught in U.S. Pat. No. 6,114,564 which is incorporated herein by reference.
It has also been discovered that an improvement in reaction rate can be achieved by pretreating the organolead compound with phenol in the presence of a tetraalkylammonium bromide, such pretreatment being performed at temperatures in the range of about 30-90xc2x0 C. for at least about 8 hours and preferably for at least about 10 hours.
Other compounds may also be present in the catalyst composition. Compounds of cerium, cobalt, copper, titanium and manganese are particularly useful, with the 2,4-pentanedionates (the oxy-2,4-pentanedionate in the case of titanium) frequently being preferred.
In addition to the aforementioned reactants and catalyst system, it is strongly preferred for a desiccant to be present in the reaction system. The preferred desiccants are non-reactive materials such as molecular sieves, as illustrated by 3-xc3x85ngstrom (hereinafter xe2x80x9c3Axe2x80x9d) molecular sieves. They are usually isolated from the other reactants, as by presence in a basket mounted to a stirrer shaft or the like.
Component A is most often present in the amount of about 0.1-10,000 ppm by weight of the appropriate Group 8, 9, or 10 metal (usually palladium), based on hydroxyaromatic compound, and component B in the amount of about 1-2,000 moles per gram-atom of the Group 8, 9, or 10 metal of component A. Component C is generally present in the amount of about 0.2-200 gram-atoms of lead per gram-atom of the Group 8, 9, or 10 metal of component A. Compounds of other metals, when present, will usually be at levels in the range of about 1-50 gram-atoms per gram-atom of the Group 8, 9, or 10 metal of component A.
The method of the invention is preferably conducted in a reactor in which the hydroxyaromatic compound and catalyst system are charged under pressure of carbon monoxide and oxygen and heated. The reaction pressure is most often within the range of about 1-500 and preferably about 1-150 atm. Gas is usually supplied in proportions of about 1-50 mole percent oxygen with the balance being carbon monoxide and optionally one or more inert gases, and in any event outside the explosion range for safety reasons. The gases may be introduced separately or as a mixture. Reaction temperatures in the range of about 60-150xc2x0 C. are typical. It is often preferred to maintain a substantially constant gas pressure and partial pressure of carbon monoxide and oxygen until conversion of the hydroxyaromatic compound is complete, as described, for example, in U.S. Pat. No. 5,399,734, which is incorporated herein by reference.
The diaryl carbonates produced by the method of the invention may be isolated by conventional techniques. It is often preferred to form and thermally crack an adduct of the diaryl carbonate with the hydroxyaromatic compound, as described in U.S. Pat. Nos. 5,239,106 and 5,312,955, which are incorporated herein by reference.